Recent clinical trial results suggest that pre-exposure prophylaxis (PrEP), a potential biomedical intervention for HIV prevention, protects against HIV infection. However, several barriers to widespread PrEP implementation have been identified. In response to some of these challenges, many researchers argue that PrEP should be delivered to high-risk populations within integrated HIV prevention programs that consist of biomedical, behavioral, and structural interventions. In the United States, men who have sex with men (MSM) remain the most highly HIV affected population, and thus should be prioritized for PrEP delivery. However, a better understanding of sexual risk behaviors among MSM over time is needed to inform the development of such programs. To achieve this goal, the proposed research project will use data collected on a cohort of adult MSM followed for one year by the Network for AIDS Research in Los Angeles Project 2: Enrollment in a Research Registry for Microbicide Clinical Trials (NARLA2). The proposed research project will examine patterns in sexual risk behaviors among MSM over time to determine the duration of high-risk behaviors within this population. Further, the proposed research will identify distinct sub-groups within thi population defined by the patterns of risk they follow over time, as well as characteristics associated with sub-group membership. Of particular interest will be the role of drug use in predicting sub-group membership. Further, the proposed research project will extend earlier applications of the Social Action Theory, which posits that self-protective behaviors are regulated by both individual and social-contextual factors, to the study of risk behaviors among MSM. This analysis will focus on the relationship between partnership characteristics and unprotected receptive anal intercourse among MSM across multiple partnerships over time. Finally, the proposed research will characterize the nature of sero-discordant partnerships among MSM. Thus, the proposed research project will yield critical information for the design of comprehensive HIV prevention packages for MSM in the United States. PUBLIC HEALTH RELEVANCE: In 2009, an estimated 42,011 individuals were diagnosed with HIV infection in the United States. However, HIV continues to disproportionately affect men who have sex with men (MSM), who accounted for 57% of all new diagnoses in that year. The proposed research project seeks to understand risk factors for HIV infection among MSM in the United States in order to inform the implementation of promising new HIV prevention strategies and reduce the burden of HIV within this population.